arXiv subjects

Pablo Peñil

Publications and source records attributed to Pablo Peñil.

4 recordsLinked to original sources

arXiv2026

Evidence for a bursty $γ$-ray QPO in the neutrino-associated FSRQ PKS 1424$-$418 using Fast Template Periodograms and Machine Learning

Standard frequency-domain searches for quasi-periodic oscillations (QPOs) in active galactic nuclei generally assume sinusoidal variability. However, $γ$-ray blazar emission often shows asymmetric flares and localized bursts, causing spectral leakage that can reduce the sensitivity of conventional methods to non-sinusoidal periodic signals. We analyze a blind sample of 100 high-cadence (7-day binned) Fermi-LAT blazar light curves. After baseline detrending with Singular Spectrum Analysis, we apply the Fast Template Periodogram using three template families: a sinusoid, a Gaussian burst, and an empirical asymmetric template derived from PG 1553+113. Candidate signals are then evaluated with a Random Forest classifier trained on $5\times10^4$ simulated light curves to distinguish genuine periodic phase structure from stochastic red noise. We identify nine $>3σ$ periodicity candidates. Three persistent QPOs, including PG 1553+113, are recovered by all template models, whereas six additional candidates are strongly suppressed under the standard sinusoidal assumption but become detectable with morphology-aware templates. Machine learning validation rejects five of these as lacking stable phase coherence. The remaining candidate, PKS 1424$-$418, shows a structurally stable 4.8-year periodicity with a source-specific significance of $3.76σ$ (approximately $2.4σ$ after accounting for the trial factor of the 100-source sample). These results demonstrate that morphology-aware periodograms, combined with structural machine learning validation, improve the detection of burst-dominated QPOs that are difficult to identify with traditional harmonic searches.

astro-ph.HE↗

arXiv2025

Decade-long periodicity study of 2FHL blazars with historical optical data

In our recent investigation, we utilized a century's worth of archival optical data to search for a decade-long periodicity from the blazar PG 1553+113, finding a hint of a 22-yr period. Building on this foundation, the current study extends our analysis to include 10 blazars from the Fermi-Large Area Telescope Second Catalog of Hard Sources (2FHL) catalogue to uncover similar long-term periodic behaviour. To ensure the reliability of our findings, we consider the impact of observational limitations, such as temporal gaps and uneven sampling, which could potentially introduce artefacts or false periodic signals. Our analysis initially identifies decade-scale periodicity in four of these blazars (AP Librae, MKN 421, MKN 501, PG 1246+586). However, further investigation reveals that three of these are likely influenced by noise and poor sampling. The most promising candidate, approximately 51 $\pm$ 9 yr signal in MKN 421, corresponds to fewer than three full cycles and cannot be considered significant. Furthermore, global significance suggests none of the candidate periodicities meet the threshold for statistical significance. These results underscore the importance of accounting for sampling artefacts and highlight the need for robust methodologies in long-term periodicity searches.

astro-ph.HE↗

arXiv2025

Distortions in Periodicity Analysis of Blazars: The Impact of Flares

Blazars, a unique class of active galactic nuclei, exhibit highly variable emission across the electromagnetic spectrum. This variability frequently manifests as intense flaring events, sparking an ongoing debate in recent literature about whether these flares exhibit periodic behavior in certain sources. However, many blazars also show clear signs of stochastic, uncorrelated flares that do not follow a regular pattern. This paper explores how the presence of one such of these stochastic flares can distort an intrinsically periodic pattern of emission in blazars. Our results demonstrate that, depending on the specific circumstances, the deviations in significance and periods can exceed 100\%. Sometimes, these deviations can be so severe that they eliminate any evidence of a periodic pattern. These findings highlight the dramatic impact that flares can have on periodicity searches. To confront this challenge, we propose an innovative approach, the Singular Spectrum Analysis method, which appears more robust against the effects of flares. As an alternative solution, we also propose the sigma clipping technique to mitigate the impact of flares. This framework offers a valuable foundation for analyzing periodicity in similar astrophysical sources that are also subject to stochastic flaring events.

astro-ph.HE↗

arXiv2017

A Trigger Interface Board to manage trigger and timing signals in CTA Large-Sized Telescope and Medium-Sized Telescope cameras

One of the main goals of the Cherenkov Telescope Array (CTA) observatory is to improve the $γ$-ray detection sensitivity by an order of magnitude, compared to the current ground-based observatories. Widening the energy coverage down to 20 GeV and up to 300 TeV is also an important goal. This goal will be possible by using Large-Sized Telescopes (LSTs) for the energy range of 20--200 GeV, Medium-Sized Telescopes (MSTs) for 100 GeV--10 TeV, and Small-Sized Telescopes (SSTs) for energies above 5 TeV. The LSTs, which focus on the lowest energies, are operated in a region dominated by background events originated from the night sky background. To reduce such background events as much as possible, the LST cameras are only read out if at least two of them have been triggered in a short-time coincidence window. Such trigger is implemented for each LST camera in a dedicated module called Trigger Interface Board (TIB). In addition, the TIB is also used in MSTs equipped with the NectarCAM camera system to manage the different trigger and timing signals between LSTs and MSTs, as well as to monitor the different counting rates and dead-time of the cameras. It also assigns a time stamp to each event, which is recorded along with the information provided by the CTA global timing distribution system, based on the White Rabbit protocol. Therefore, the event arrival time can be determined in a redundant way. In this contribution, the main features and the technical performance of the TIB are presented.

astro-ph.IM↗